This invention is directed to an improved method and apparatus for using a non-contact mask in a laser system for defining and removing very small, highly detailed image patterns from a polyimide substrate. More particularly it is directed to an imaging system where both the work piece and the mask are moved in the laser beam to allow laser radiation to cover a pattern on the mask that has an area which substantially exceeds the area of the laser beam.
During dual access flex circuit production, a desired conductor pattern is etched into a work piece which is a prepared copper-clad polyimide material such as Dupont's Kapton brand polyimide film, other adhesives or acrylics, photoresists or thin film metals. Typically, protective and base dielectric layers containing drilled or punched holes are laminated to the conductive trace layers to allow access to the leader pads. A polyimide substrate supports the trace layers during the etching and cleaning processes, preventing the trace layers from bending. Later, additional etching, stripping and cleaning processes remove the polyimide substrate supports. However, there are two drawbacks to this process. First, the drilling and punching techniques frequently result in imperfect image patterns (jagged edges, stringers, etc.) for finely patterned features. Second, the etching, stripping and cleaning processes require increased handling which bends and damages the delicate leads and traces.
Consequently, excimer lasing with mask imaging techniques was developed to define and remove very small detailed image patterns from a polyimide substrate. The ablation process is ideal for this highly precise process because it is non-thermal and can be adjusted to remove a polyimide substrate to an exact depth. Currently there are many mask imaging techniques used with an excimer laser process to remove a polyimide substrate. The features and limitations of the prior processes are described below.
The contact conformal mask method involved fanning the laser beam over a mask placed in contact with a polyimide substrate. This method, which is referred to in U.S. Pat. No. 4,764,485, Loughran, et al, has several drawbacks. First, the work piece pattern size and shape depends on the object mask size; that is, the resulting work piece pattern can be no better defined than the machined (or etched) object mask pattern. Second, contamination may be trapped between the mask and substrate. Third, the mask restricts inspection of the image zone on the work piece surface, retarding quality assessment during the imaging process. Fourth, if the contact mask and the work piece do not conform exactly, the image tends to blur. Fifth, use of a contact mask requires manual alignment, which increases labor costs.
In the prior art there are also two non-contact stationary mask methods. The first method involves directing an excimer laser beam through a stationary object mask and stationary focusing lens, which is positioned between the mask and the work piece surface, a polyimide substrate. This concept is discussed in U.S. Pat. No. 4,724,219, Ridinger. Thus, the object mask pattern is magnified to create finely detailed image features on the substrate that are a magnification factor smaller than the details producible on the object mask. The second non-contact method involves directing an excimer laser beam through a focusing lens first, then through a stationary object mask and onto a polyimide substrate. This method is discussed in U.S. Pat. No. 4,786,358, Yamazaki. Thus the object mask pattern is not magnified on the substrate.
Both of the non-contact methods reduce contamination and make it easier to access the work piece surface during the imaging process. However, both have drawbacks when compared to the non-contact moving mask. First, the projected image size in a non-contact stationary mask is confined to the cross-sectional area of the laser beam. Second, there is an increased chance of damaging the mask from continual impact of the laser beam on one location of the mask.
In the prior art there is also a technique which uses a non-contact mask which can be moved when the laser is not activated. Such a non-contact mask method with a movable mask held stationary when lasing commences provides results which are improved over the non-contact stationary mask method because the size of the work piece image is not limited by the size of the cross-sectional area of the laser beam. However, there are several drawbacks to this prior art method as well.
First, because the mask cannot move during the lasing process, the laser must be deactivated and the mask repositioned before lasing can resume. Consequently, to ensure that the image pattern on the mask, and hence, the work piece surface, receives 100% ablative coverage, the beam is overlapped on the image pattern of the mask and work piece surfaces. Excessive energy input from the laser beam at these overlap areas may damage the mask and lead to excessive energy applied to the work piece surface. Second, the process of halting the laser and repositioning the mask is also more time consuming.
Finally, two prior art non-contact "moving" mask methods are briefly described in "Why Excimer Lasers Excel in Marking", by Sercel, et al. The first method involves directing a laser beam through a mask that moves along one axis (left to right), and projects the image onto a counter-correlated moving target surface. The second method does not actually use a "moving" mask; however, the effect is similar. The second method involves directing a laser beam through a turning mirror that moves along one axis (up and down) in front of a stationary mask, so the beam scans over the mask, projecting the mask object image onto a work piece. However, there are several drawbacks to both methods. First, for both methods there is only one axis of movement (left or right for the first method, and up and down for the second method). In the second method, the single axis of movement and thus the projected image size, is limited by the diameter of the imaging lens.